In making lithographic printing plates, a step of coating a desensitizing gum, called a gumming-up step, is provided as a final step for protecting non-image areas (areas which retain water in order to repel a printing ink).
The purposes of applying the desensitizing gum to non-image areas are not only to protect the hydrophilic character of the non-image areas but also to protect the non-image areas from being stained or flawed by adhesion of fingerprints, fats and oils, dusts, etc. upon correction of image areas such as retouching or erasure; during storage before printing and after plate making or before re-use; and, upon handling to mount the printing plate on a press. In addition, application of desensitizing gum acts to suppress oxidative stains.
As the gum composition for lithographic printing plates, compositions comprising an aqueous solution of gum arabic, cellulose gum or a water-soluble high molecular weight substance containing carboxy groups in the molecule and optionally containing a pH-adjusting agent, an antiseptic, etc. have so far been popularly used.
However, these conventionally known compositions have a disadvantage in that the compositions can cause image blinding at initial press. In the final step of finishing a printing plate, a gum solution is applied to the printing plate and spread all over the plate surface using a sponge or cotton pad. This step is followed by polishing the plate surface with a cotton pad or a cloth wipe until the surface dries. Upon this surface a water-soluble high molecular weight substance is thickly coated, in part on image areas (areas which receive an ink). The thickly coated image areas have such a poor ink receptivity in printing that many copies must be printed before the image fully accepts ink. Where this phenomenon, generally called image blinding at initial press, takes place, generally the plate must be subjected to a step of washing with water or with a weakly acidic solution to thereby remove the hydrophilic colloid absorbing on the image areas. This washing step consumes considerable time, and in attempts to make this time consuming step easier, a removing solution for gum has been developed as described in U.S. Pat. No. 4,024,085.
Alternatively, image areas can be coated with fats and oils before the gumming-up step for the purpose of protecting the ink-receptive properties of the image areas. However, this makes the plate-making step complicated and reduces workability. In addition, disposal of waste liquor can aggravate pollution and be a health hazard.
Water-soluble organic high molecular weight compounds which do not cause image blinding also have been developed as desensitizing gums. For example, U.S. Pat. No. 4,095,525 and British Pat. No. 2,010,298, West German Pat. No. 2,504,594, and Soviet Pat. No. 623,755 disclose dextrin, pullulan and its derivatives, carboxy-containing polyacrylamide derivatives, methyl acrylate- or methacrylate-grafted polyacrylamide copolymers, etc. However, these compounds have a disadvantage in that the desensitizing action on non-image areas is poor.